Noble gases Neon and argon: a role for the chemical patterns of multiple populations in globular clusters?

Abstract

We focus on the sodium destruction in models reaching the high hot bottom burning temperatures needed to efficiently cycle oxygen to nitrogen in AGB models at the nominal [Fe/H] of the cluster NGC 2808. We increase the initial neon abundance by a factor 2-4 with respect to the "standard" abundances obtained by scaling the solar values down to the metallicity of this cluster, and explore the average abundances in the ejecta obtained by adopting smaller mass-loss rates. Higher neon produces higher sodium in the AGB envelope. Lowering the mass-loss rate allows both to keep reasonably large sodium abundances and to increase the depletion of oxygen and magnesium. A balance between the lower mass-loss rates and the necessity of not increasing too much the episodes of third dredge up gives a neon abundance larger by a factor two and a mass-loss rate smaller by a factor four as best compromise. Comparison with the abundances in NGC 2808 shows a better agreement than the standard models for all the patterns of abundances, but the extreme stars (group E) requires models slightly less rich in iron. t Thus, we propose that the extreme population in NGC 2808 is composed of stars having a slightly smaller metallicity, and sketch a possible scenario for its formation, in the framework of the hierarchical clusters assembly scenario. Abundances of potassium are larger by $\sim 0.2 dex$ in the E group, but the explanation in terms of burning of the initial argon requires a drastic increase of the relevant cross section. The abundances of neon and argon at low metallicities may be an important tool to better reproduce the abundances of light elements in the framework of the AGB model for globular clusters.